1. Field of the Invention
The present invention relates to the field of semiconductor devices. More particularly, the present invention provides an improved device which can perform multiple functions such as imaging and tracking or multi-color imaging, especially with a first sensor operating in real time and a second sensor with functions controlled by the first sensor.
2. Description of Related Art
Charge coupled devices (hereinafter referred to as CCD's) and other charge transfer devices have been in wide use and have found particular utility in analog signal processing and as charge coupled imagers. In a simple charge coupled device, mobile charge packets (such as electrons or holes) are introduced at a first location in a silicon device. The silicon device may, for example, be a P- substrate. The charge packets can be introduced with an electrical charge injection or from electron-hole pair generation resulting from photon bombardment in a photosensing section. When the charge is generated from photon bombardment, an integration time may be provided so that a useful amount of charge is developed.
After generation, the charge is transferred out of the CCD using a series of gates along the surface of the silicon substrate. The gates are clocked to a series of voltage levels that move the charge along the semiconductor surface.
It has been proposed that such CCD's could be utilized to detect and image light of a variety of wavelength regions by forming photosensing cells adjacent to each other which alternately detect light of a first and a second wavelength. However, by forming the CCD in this fashion, the pixel density of each type of sensor is reduced and, therefore, the quality of the image that can be produced. Further, such structures would be unreliable for tracking since an IR laser spot, for example, could be focused on a visible sensing element on which it would not be detected.
Harada et al. (U.S. Pat. No. 4,651,001) disclose a CCD using two photosensing sections which are stacked. The first, upper photosensing section is an amorphous silicon layer utilized to detect visible light. Infrared light passes through the upper layer and is detected in a second, lower, Schottky diode layer. The structure is formed in this fashion to increase the pixel density and, therefore, image quality. Harada et al. use the same size pixel for both the visible and IR imaging and, further, use the same CCD multiplexer and output amplifier to read out two vastly different images (i.e., the visible and the IR).
The structure proposed by Harada et al. creates a wide variety of problems. For example, since Harada et al. utilize a combination of a Schottky diode and an amorphous silicon layer, it is necessary to provide a dramatic temperature gradient between the two layers. Specifically, the amorphous silicon layer must be maintained at near ambient conditions to prevent trapping of charges at low temperatures. Conversely, the Schottky diode portion must be maintained at or near liquid nitrogen temperatures in order to control the flow of dark current.
Consequently, a structure such as that proposed in FIG. 8 of Harada et al. must be provided. In such structure, the Schottky diode portion is mounted on a first side of a substrate and exposed to liquid nitrogen. The amorphous silicon layer is swept with dry nitrogen gas in order to maintain a large thermal gradient in the structure. It would be extremely difficult or impossible to implement a structure which could withstand such extreme temperature gradients reliably. Further, no band pass filter is used between the visible and IR portion. IR absorption occurs in the amorphous silicon layer and degrades the quality of the IR image and, in some wavelengths, opaque the IR. The structure provided by Harada et al. may, therefore, only be useful in a narrow band of interest. Also, near IR radiation is transmitted to the Schottky barrier divide, potentially causing the IR image to bloom. Still further, by using the same multiplexer and output amplifier for both the silicon amorphous layer and the Schottky diode portion, implementation would be impractical since the dynamic range and the charge handling capacity of both signals differ widely and simultaneous use of the two layers would not be possible. Still further, the structure provided by Harada et al. would be difficult to manufacture because it could require approximately 15 to 20 masking layers.
Roshen, U.S. Pat. No. 3,962,578, discloses a single element, two detector structure. The first detector is transparent to the wavelength of the inner detector. Both detectors are maintained at low temperature, the upper element sensing long wavelength, 1.4 .mu.m light and the lower element detecting shorter wavelength 4.4 .mu.m light. The first element may, for example, be an indium antimonide P-N junction chip, while the lower element is a germanium or silicon planar P-N junction chip. Both elements are read out by the same circuit and the same pixel size is used for the visible and the IR.
Roshen does not show or suggest a focal plane array, i.e., an array of sensors that could perform imaging in an x-y direction in one or more different frequencies over a wide field of view. No provision for sampling a single wavelength range of interest in the upper chip, while imaging the wavelength range of interest in the lower chip is shown or suggested.
Other patents discussing multi-function CCD's include Lilliquist (U.S. Pat. Nos. 4,751,571 and 4,679,068), in which an incoming beam of light is split, Schnitzler (U.S. Pat. No. 4,679,068) and Roosild et al. (U.S. Pat. No. 3,902,066).
Neither Harada et al. nor the remaining prior art show or suggest a method of effectively preventing sunlight from flooding a thermal imager. Further, the prior art does not show or suggest a method of simultaneously imaging within two spectral bands, especially with sensors operating at different frequencies or frame rates. Further, the prior art does not show or suggest two Schottky barrier focal plane arrays for simultaneous imaging in two different wavelengths using staggered or semi-transparent Schottky barrier detectors, especially using two independent CCD multiplexers and amplifiers. No provision for the operation of the two planes at different speeds or readout rates is shown or suggested.
From the above it is seen that an improved multi-mode sensor is needed. In particular it is desirable to provide a sensor in which a first layer can be used for detection, communication, and tracking while a second is used for simultaneous imaging. Further, it is desirable to provide a multi-mode sensor with improved element geometry to provide a combined imaging and laser tracking function.